MERIE plasma reactor with overhead RF electrode tuned to the plasma with arcing suppression

ABSTRACT

A plasma reactor for processing a semiconductor workpiece, includes reactor chamber having a chamber wall and containing a workpiece support for holding the semiconductor support, the electrode comprising a portion of the chamber wall, an RF power generator for supplying power at a frequency of the generator to the overhead electrode and capable of maintaining a plasma within the chamber at a desired plasma ion density level. The overhead electrode has a capacitance such that the overhead electrode and the plasma formed in the chamber at the desired plasma ion density resonate together at an electrode-plasma resonant frequency, the frequency of the generator being at least near the electrode-plasma resonant frequency. The reactor further includes a set of MERIE magnets surrounding the plasma process area overlying the wafer surface that produce a slowly circulating magnetic field which stirs the plasma to improve plasma ion density distribution uniformity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/007,367, filed Oct. 22, 2001 by Daniel Hoffman et al., entitled“MERIE PLASMA REACTOR WITH OVERHEAD RF ELECTRODE TUNED TO THE PLASMAWITH ARCING SUPPRESSION”, which is a continuation-in-part of U.S.application Ser. No. 09/527,342, filed Mar. 17, 2000 now U.S. Pat. No.6,528,751 by Daniel Hoffman, et al., entitled, “PLASMA REACTOR WITHOVERHEAD RF ELECTRODE TUNED TO THE PLASMA” and assigned to the presentassignee.

BACKGROUND OF THE INVENTION

An RF plasma reactor is used to process semiconductor wafers to producemicroelectronic circuits. The reactor forms a plasma within a chambercontaining the wafer to be processed. The plasma is formed andmaintained by application of RF plasma source power coupled eitherinductively or capacitively into the chamber. For capacitive coupling ofRF source power into the chamber, an overhead electrode (facing thewafer) is powered by an RF source power generator.

One problem in such reactors is that the output impedance of the RFgenerator, typically 50 Ohms, must be matched to the load impedancepresented by the combination of the electrode and the plasma. Otherwisethe amount of RF power delivered to the plasma chamber will fluctuatewith fluctuations in the plasma load impedance so that certain processparameters such as plasma density cannot be held within the requiredlimits. The plasma load impedance fluctuates during processing becauseit depends upon conditions inside the reactor chamber which tend tochange dynamically as processing progresses. At an optimum plasmadensity for dielectric or conductor etch processes, the load impedanceis very small compared to the output impedance of the RF generator andcan vary significantly during the processing of the wafer. Accordingly,an impedance match circuit must be employed to actively maintain animpedance match between the generator and the load. Such activeimpedance matching uses either a variable reactance and/or a variablefrequency. One problem with such impedance match circuits is that theymust be sufficiently agile to follow rapid changes in the plasma loadimpedance, and therefore are relatively expensive and can reduce systemreliability due to their complexity.

Another problem is that the range of load impedances over which thematch circuit can provide an impedance match (the “match space”) islimited. The match space is related to the system Q, where Q=Δf/f, fbeing a resonant frequency of the system and Δf being the bandwidth oneither side of f within which resonant amplitude is within 6 dB of thepeak resonant amplitude at f. The typical RF generator has a limitedability to maintain the forward power at a nearly constant level even asmore RF power is reflected back to the generator as the plasma impedancefluctuates. Typically, this is achieved by the generator servoing itsforward power level, so that as an impedance mismatch increases (andtherefore reflected power increases), the generator increases itsforward power level. Of course, this ability is limited by the maximumforward power which the generator is capable of producing. Typically,the generator is capable of handling a maximum ratio of forward standingwave voltage to reflected wave voltage (i.e., the voltage standing waveratio or VSWR) of not more than 3:1. If the difference in impedancesincreases (e.g., due to plasma impedance fluctuations during processing)so that the VSWR exceeds 3:1, then the RF generator can no longercontrol the delivered power, and control over the plasma is lost. As aresult, the process is likely to fail. Therefore, at least anapproximate impedance match must be maintained between the RF generatorand the load presented to it by the combination of the electrode and thechamber. This approximate impedance match must be sufficient to keep theVSWR at the generator output within the 3:1 VSWR limit over the entireanticipated range of plasma impedance fluctuations. The impedance matchspace is, typically, the range of load impedances for which the matchcircuit can maintain the VSWR at the generator output at or below 3:1.

A related problem is that the load impedance itself is highly sensitiveto process parameters such as chamber pressure, plasma source powerlevel, source power frequency and plasma density. This limits the rangeof such process parameters (the “process window”) within which theplasma reactor must be operated to avoid an unacceptable impedancemismatch or avoid fluctuations that take load impedance outside of thematch space. Likewise, it is difficult to provide a reactor which can beoperated outside of a relatively narrow process window and process use,or one that can handle many process applications.

Another related problem is that the load impedance is also affected bythe configuration of the reactor itself, such as dimensions of certainmechanical features and the conductivity or dielectric constant ofcertain materials within the reactor. (Such configurational items affectreactor electrical characteristics, such as stray capacitance forexample, that in turn affect the load impedance.) This makes itdifficult to maintain uniformity among different reactors of the samedesign due to manufacturing tolerances and variations in materials. As aresult, with a high system Q and correspondingly small impedance matchspace, it is difficult to produce any two reactors of the same designwhich exhibit the same process window or provide the same performance.

Another problem is inefficient use of the RF power source. Plasmareactors are known to be inefficient, in that the amount of powerdelivered to the plasma tends to be significantly less than the powerproduced by the RF generator. As a result, an additional cost ingenerator capability and a trade-off against reliability must beincurred to produce power in excess of what is actually required to bedelivered into the plasma.

SUMMARY OF THE INVENTION

A plasma reactor for processing a semiconductor workpiece, includes areactor chamber having a chamber wall and containing a workpiece supportfor holding the semiconductor workpiece, an overhead electrode overlyingsaid workpiece support, the electrode comprising a portion of thechamber wall, an RF power generator for supplying power at a frequencyof the generator to the overhead electrode and capable of maintaining aplasma within the chamber at a desired plasma ion density level. Theoverhead electrode has a capacitance such that the overhead electrodeand the plasma formed in the chamber at the desired plasma ion densityresonate together at an electrode-plasma resonant frequency, thefrequency of the generator being at least near the electrode-plasmaresonant frequency. The reactor further includes a set of MERIE magnetssurrounding the plasma process area overlying the wafer surface thatproduce a slowly circulating magnetic field which stirs the plasma toimprove plasma ion density distribution uniformity. The reactor caninclude an insulating layer formed on a surface of the overheadelectrode facing the workpiece support, a capacitive insulating layerbetween the RF power generator and the overhead electrode, and a metalor ceramic foam layer overlying and contacting a surface of the overheadelectrode that faces away from the workpiece support. The insulatinglayer provides a capacitance sufficient to suppress arcing within thegas injection ports, the capacitive insulating layer has a sufficientcapacitance to block D.C. current from a plasma within the chamber fromflowing through the overhead electrode, and the metal foam layer is of asufficient thickness to suppress an axial electric field within the gasinjection orifices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut-away cross-sectional side view of a plasma reactoraccording to an embodiment of the invention.

FIGS. 2A and 2B are diagrams illustrating, respectively, the coaxialstub of FIG. 1 and the voltage and current standing wave amplitudes as afunction of position along the coaxial stub.

FIG. 3 illustrates the subtraction of current at the input power tappoint on the coaxial stub that occurs in response to high plasma loadimpedance to maintain a more constant delivered VHF power level in alarger match space.

FIG. 4 illustrates the addition of current at the input power tap pointon the coaxial stub that occurs in response to low plasma load impedanceto maintain a more constant delivered VHF power level in a larger matchspace.

FIG. 5 is a graph illustrating the low-Q reflection coefficient as afunction of frequency of the embodiment of FIG. 1.

FIG. 6 is a graph illustrating the interaction of the currentcontribution at the input power tap point on the coaxial stub with thestanding wave current and voltage along the stub length.

FIG. 7 illustrates an alternative embodiment of the coaxial stub of FIG.1.

FIG. 8 depicts another embodiment of the present invention.

FIG. 9 is an enlarged view corresponding to FIG. 8.

FIG. 10 is an enlarged view of FIG. 9.

FIG. 11A is another enlarged view of FIG. 8.

FIG. 11B depicts an alternative embodiment corresponding to FIG. 11A.

FIG. 12 depicts yet another embodiment of the present invention.

FIG. 13 is a top view corresponding to FIG. 12.

FIG. 14 is a top view corresponding to an alternate embodiment of thereactor of FIG. 13.

DETAILED DESCRIPTION

Referring to FIG. 1, a plasma reactor includes a reactor chamber 100with a wafer support 105 at the bottom of the chamber supporting asemiconductor wafer 110. A process kit may include, in an examplaryimplementation, a conductive or semi-conductive ring 115 supported by adielectric ring 120 on a grounded chamber body 127. The chamber 100 isbounded at the top by a disc shaped overhead conductive electrodesupported at a predetermined gap length above the wafer 110 on groundedchamber body 127 by a dielectric seal. The overhead electrode 125 may bea metal (e.g., aluminum) which may be covered with a semi-metal material(e.g., Si or SiC) on its interior surface, or it may be itself asemi-metal material. An RF generator 150 applies RF power to theelectrode 125. RF power from the generator 150 is coupled through acoaxial cable 162 matched to the generator 150 and into a coaxial stub135 connected to the electrode 125. The stub 135 has a characteristicimpedance, resonance frequency, and provides an impedance match betweenthe electrode 125 and the coaxial cable 162 or the output of the RFpower generator 150, as will be more fully described below. The chamberbody is connected to the RF return (RF ground) of the RF generator 150.The RF path from the overhead electrode 125 to RF ground is affected bythe capacitance of the dielectric seal 120 and by the capacitance of thedielectric seal 130. The wafer support 105, the wafer 110 and theprocess kit conductive or semiconductive ring 115 provide the primary RFreturn path for RF power applied to the electrode 125.

The capacitance of the overhead electrode assembly 126, including theelectrode 125, the process kit 115, 120 and the dielectric seal 130measured with respect to RF return or ground is, in an exemplaryembodiment, 180 pico farads. The electrode assembly capacitance isaffected by the electrode area, the gap length (distance between wafersupport and overhead electrode), and by factors affecting straycapacitances, especially the dielectric values of the seal 130 and ofthe dielectric ring 120, which in turn are affected by the dielectricconstants and thicknesses of the materials employed. More generally, thecapacitance of the electrode assembly 126 (an unsigned number or scalar)is equal or nearly equal in magnitude to the negative capacitance of theplasma (a complex number) at a particular source power frequency, plasmadensity and operating pressure, as will be discussed below.

Many of the factors influencing the foregoing relationship are in greatpart predetermined due to the realities of the plasma processrequirements needed to be performed by the reactor, the size of thewafer, and the requirement that the processing be carried out uniformlyover the wafer. Thus, the plasma capacitance is a function of the plasmadensity and the source power frequency, while the electrode capacitanceis a function of the wafer support-to-electrode gap (height), electrodediameter, and dielectric values of the insulators of the assembly.Plasma density, operating pressure, gap, and electrode diameter mustsatisfy the requirements of the plasma process to be performed by thereactor. In particular, the ion density must be within a certain range.For example, silicon and dielectric plasma etch processes generallyrequire the plasma ion density to be within the range of 10⁹-10¹²ions/cc. The wafer electrode gap provides an optimum plasma iondistribution uniformity for 8 inch wafers, for example, if the gap isabout 2 inches. The electrode diameter is preferably at least as greatas, if not greater than the diameter of the wafer. Operating pressuressimilarly have practical ranges for typical etch and other plasmaprocesses.

But it has been found that other factors remain which can be selected toachieve the above preferred relationship, particularly choice of sourcefrequency and choice of capacitances for the overhead electrode assembly126. Within the foregoing dimensional constraints imposed on theelectrode and the constraints (e.g., density range) imposed on theplasma, the electrode capacitance can be matched to the magnitude of thenegative capacitance of the plasma if the source power frequency isselected to be a VHF frequency, and if the dielectric values of theinsulator components of electrode assembly 126 are selected properly.Such selection can achieve a match or near match between source powerfrequency and plasma-electrode resonance frequency.

Accordingly in one exemplary embodiment, for an 8-inch wafer theoverhead electrode diameter is approximately 11 inches, the gap is about2 inches, the plasma density and operating pressure is typical for etchprocesses as above-stated, the VHF source power frequency is 210 MHz(although other VHF frequencies could be equally effective), and thesource power frequency, the plasma electrode resonance frequency and thestub resonance frequency are all matched or nearly matched.

More particularly, these three frequencies are slightly offset from oneanother, with the source power frequency being 210 MHz, theelectrode-plasma resonant frequency being approximately 200 MHz, and thestub frequency being about 220 MHz, in order to achieve a de-tuningeffect which advantageously reduces the system Q. Such a reduction insystem Q renders the reactor performance less susceptible to changes inconditions inside the chamber, so that the entire process is much morestable and can be carried out over a far wider process window.

The coaxial stub 135 is a specially configured design which furthercontributes to the overall system stability, its wide process windowcapabilities, as well as many other valuable advantages. It includes aninner cylindrical conductor 140 and an outer concentric cylindricalconductor 145. An insulator 147 (denoted by cross-hatching in FIG. 1)for example having a relative dielectric constant of 1 fills the spacebetween the inner and outer conductors 140, 145. The inner and outerconductors 140, 145 may be formed, for example, of nickel-coatedaluminum. In an exemplary embodiment, the outer conductor 145 has adiameter of about 4 inches and the inner conductor 140 has a diameter ofabout 1.5 inches. The stub characteristic impedance is determined by theradii of the inner and outer conductors 140, 145 and the dielectricconstant of the insulator 147. The stub 135 of the embodiment describedabove has a characteristic impedance of 65Ω. More generally, the stubcharacteristic impedance exceeds the source power output impedance byabout 20%-40% and preferably by about 30%. The stub 135 has an axiallength of about 29 inches—a quarter wavelength at 220 MHz—in order tohave a resonance in the vicinity of 220 MHz to generally match whilebeing slightly offset from the VHF source power frequency of 210 MHz.

A tap 160 is provided at a particular point along the axial length ofthe stub 135 for applying RF power from the RF generator 150 to the stub135, as will be discussed below. The RF power terminal 150 b and the RFreturn terminal 150 a of the generator 150 are connected at the tap 160on the stub 135 to the inner and outer coaxial stub conductors 140, 145,respectively. These connections are made via a generator-to-stub coaxialcable 162 having a characteristic impedance that matches the outputimpedance of the generator 150 (typically, 50 Ω) in the well-knownmanner. A terminating conductor 165 at the far end 135 a of the stub 135shorts the inner and outer conductors 140, 145 together, so that thestub 135 is shorted at its far end 135 a. At the near end 135 b (theunshorted end) of the stub 135, the outer conductor 145 is connected tothe chamber body via an annular conductive housing or support 175, whilethe inner conductor 140 is connected to the center of electrode 125 viaa conductive cylinder or support 176. A dielectric ring 180 is heldbetween and separates the conductive cylinder 176 and the electrode 125.

The inner conductor 140 provides a conduit for utilities such as processgases and coolant. The principal advantage of this feature is that,unlike typical plasma reactors, the gas line 170 and the coolant line173 do not cross large electrical potential differences. They thereforemay be constructed of metal, a less expensive and more reliable materialfor such a purpose. The metallic gas line 170 feeds gas inlets 172 in oradjacent the overhead electrode 125 while the metallic coolant line 173feeds coolant passages or jackets 174 within the overhead electrode 125.

An active and resonant impedance transformation is thereby provided bythis specially configured stub match between the RF generator 150, andthe overhead electrode assembly 126 and processing plasma load,minimizing reflected power and providing a very wide impedance matchspace accommodating wide changes in load impedance. Consequently, wideprocess windows and process flexibility is provided, along withpreviously unobtainable efficiency in use of power, all while minimizingor avoiding the need for typical impedance match apparatus. As notedabove, the stub resonance frequency is also offset from ideal match tofurther enhance overall system Q, system stability and process windowsand multi-process capability.

Matching the Electrode-Plasma Resonance Frequency and the VHF SourcePower Frequency:

As outlined above, a principal feature is to configure the overheadelectrode assembly 126 for resonance with the plasma at theelectrode-plasma resonant frequency and for the matching (or the nearmatch of) the source power frequency and the electrode-plasma frequency.The electrode assembly 126 has a predominantly capacitive reactancewhile the plasma reactance is a complex function of frequency, plasmadensity and other parameters. (As will be described below in greaterdetail, a plasma is analyzed in terms of a reactance which is a complexfunction involving imaginary terms and generally corresponds to anegative capacitance.) The electrode-plasma resonant frequency isdetermined by the reactances of the electrode assembly 126 and of theplasma (in analogy with the resonant frequency of a capacitor/inductorresonant circuit being determined by the reactances of the capacitor andthe inductor). Thus the electrode-plasma resonant frequency may notnecessarily be the source power frequency, depending as it does upon theplasma density. The problem, therefore, is to find a source powerfrequency at which the plasma reactance is such that theelectrode-plasma resonant frequency is equal or nearly equal to thesource power frequency, given the constraints of practical confinementto a particular range of plasma density and electrode dimensions. Theproblem is even more difficult, because the plasma density (whichaffects the plasma reactance) and the electrode dimensions (which affectelectrode capacitance) must meet certain process constraints.Specifically, for dielectric and conductor plasma etch processes, theplasma density should be within the range of 10⁹-10¹² ions/cc, which isa constraint on the plasma reactance. Moreover, a more uniform plasmaion density distribution for processing 8-inch diameter wafers forexample, is realized by a wafer-to-electrode gap or height of about 2inches and an electrode diameter on the order of the wafer diameter, orgreater, which is a constraint on the electrode capacitance. On theother hand, a different gap may be utilized for a 12-inch diameterwafer.

Accordingly, by matching (or nearly matching) the electrode capacitanceto the magnitude of the negative capacitance of the plasma, theelectrode-plasma resonant frequency and the source power frequency areat least nearly matched. For the general conductor and dielectric etchprocess conditions enumerated above (i.e., plasma density between10⁹-10¹² ions/cc, a 2-inch gap and an electrode diameter on the order ofroughly 11 inches), the match is possible if the source power frequencyis a VHF frequency. Other conditions (e.g., different wafer diameters,different plasma densities, etc.) may dictate a different frequencyrange to realize such a match in carrying out this feature of thereactor. As will be detailed below, under favored plasma processingconditions for processing 8-inch wafers in several principalapplications including dielectric and metal plasma etching and chemicalvapor deposition, the plasma capacitance in one typical working examplehaving plasma densities as set forth above was between −50 and −400 picofarads. In an exemplary embodiment the capacitance of the overheadelectrode assembly 126 was matched to the magnitude of this negativeplasma capacitance by using an electrode diameter of 11 inches, a gaplength (electrode to pedestal spacing) of approximately 2 inches,choosing a dielectric material for seal 130 having a dielectric constantof 9, and a thickness of the order of one inch, and a dielectricmaterial for the ring 120 having a dielectric constant of 4 andthickness of the order of 10 mm.

The combination of electrode assembly 126 and the plasma resonates at anelectrode-plasma resonant frequency that at least nearly matches thesource power frequency applied to the electrode 125, assuming a matchingof their capacitances as just described. We have discovered that forfavored etch plasma processing recipes, environments and plasmas, thiselectrode-plasma resonant frequency and the source power frequency canbe matched or nearly matched at VHF frequencies; and that it is highlyadvantageous that such a frequency match or near-match be implemented.In an exemplary embodiment, the electrode-plasma resonance frequencycorresponding to the foregoing values of plasma negative capacitance isapproximately 200 MHz, as will be detailed below. The source powerfrequency is 210 MHz, a near-match in which the source power frequencyis offset slightly above the electrode-plasma resonance frequency inorder to realize other advantages to be discussed below.

The plasma capacitance is a function of among other things, plasmaelectron density. This is related to plasma ion density, which needs, inorder to provide good plasma processing conditions, to be kept in arange generally 10⁹ to 10¹² ions/cc. This density, together with thesource power frequency and other parameters, determines the plasmanegative capacitance, the selection of which is therefore constrained bythe need to optimize plasma processing conditions, as will be furtherdetailed below. But the overhead electrode assembly capacitance isaffected by many physical factors, e.g. gap length (spacing betweenelectrode 125 and the wafer); the area of electrode 125; the range ofthe dielectric loss tangent for the dielectric seal 130; the choice ofdielectric constant of the dielectric seal 130 between electrode 125 andgrounded chamber body 127; the choice of dielectric constant for theprocess kit dielectric seal 130; and the thickness of the dielectricseals 130 and 120 and the thickness and dielectric constant of the ring180. This permits some adjustment of the electrode assembly capacitancethrough choices made among these and other physical factors affectingthe overhead electrode capacitance. We have found that the range of thisadjustment is sufficient to achieve the necessary degree of matching ofthe overhead electrode assembly capacitance to the magnitude of thenegative plasma capacitance. In particular, the dielectric materials anddimensions for the seal 130 and ring 120 are chosen to provide thedesired dielectric constants and resulting dielectric values. Matchingthe electrode capacitance and the plasma capacitance can then beachieved despite the fact that some of the same physical factorsinfluencing electrode capacitance, particularly gap length, will bedictated or limited by the following practicalities: the need to handlelarger diameter wafers; to do so with good uniformity of distribution ofplasma ion density over the full diameter of the wafer; and to have goodcontrol of ion density vs ion energy.

Given the foregoing range for the plasma capacitance and the matchingoverhead electrode capacitance, the electrode-plasma resonance frequencywas approximately 200 MHz for a source power frequency of 210 MHz.

A great advantage of choosing the capacitance of the electrode assembly126 in this manner, and then matching the resultant electrode-plasmaresonant frequency and the source power frequency, is that resonance ofthe electrode and plasma near the source power frequency provides awider impedance match and wider process window, and consequently muchgreater immunity to changes in process conditions, and therefore greaterperformance stability. The entire processing system is rendered lesssensitive to variations in operating conditions, e.g., shifts in plasmaimpedance, and therefore more reliable along with a greater range ofprocess applicability. As will be discussed later in the specification,this advantage is further enhanced by the small offset between theelectrode-plasma resonant frequency and the source power frequency.

Why the Plasma Has a Negative Capacitance:

The capacitance of the plasma is governed by the electrical permittivityof the plasma, ε, which is a complex number and is a function of theelectrical permittivity of free space ε₀, the plasma electron frequencyω_(pe), the source power frequency w and the electron-neutral collisionfrequency n_(en) in accordance with the following equation:ε=ε₀[1−ω_(pe) ²/(ω(ω+iν _(en)))] where i=(−1)^(1/2).(The plasma electron frequency ω_(pe) is a simple function of the plasmaelectron density and is defined in well-known publications on plasmaprocessing.)

In one working example, the neutral species was Argon, the plasmaelectron frequency was about 230 MHz, the RF source power frequency wasabout 210 MHz with chamber pressure in the range of 10 mT to 200 mT withsufficient RF power applied so that the plasma density was between 10⁹and 10¹² cc⁻¹. Under these conditions, which are typical of thosefavorable to plasma etch processes, the plasma generally has a negativecapacitance because its effective electrical permittivity defined by theforegoing equation is negative. Under these conditions, the plasma had anegative capacitance of −50 to −400 pico farads. Then as we have seenabove in more general terms, the plasma capacitance, as a function ofplasma electron density (as well as source power frequency andelectron-neutral collision frequency) tends to be generally limited byfavored plasma process realities for key applications such as dielectricetch, metal etch and CVD, to certain desired ranges, and to have anegative value at VHF source power frequencies. By exploiting thesecharacteristics of the plasma, the electrode capacitance matching andfrequency-matching features of the reactor achieve a process windowcapability and flexibility and stability of operation not previouslypossible.

Impedance Transformation Provided by the Stub 135:

The stub 135 provides an impedance transformation between the 50 Ωoutput impedance of the RF generator 150 and the load impedancepresented by the combination of the electrode assembly 126 and theplasma within the chamber. For such an impedance match, there must belittle or no reflection of RF power at the generator-stub connection andat the stub-electrode connection (at least no reflection exceeding theVSWR limits of the RF generator 150). How this is accomplished will nowbe described.

At the desired VHF frequency of the generator 150 and at a plasmadensity and chamber pressure favorable for plasma etch processes (i.e.,10⁹-10¹² ions/cm³ and 10 mT-200 mT, respectively), the impedance of theplasma itself is about (0.3+(i)7)Ω, where 0.3 is the real part of theplasma impedance, i=(−1)^(1/2), and 7 is the imaginary part of theplasma impedance. The load impedance presented by the electrode-plasmacombination is a function of this plasma impedance and of thecapacitance of the electrode assembly 126. As described above, thecapacitance of the electrode assembly 126 is selected to achieve aresonance between the electrode assembly 126 and the plasma with anelectrode-plasma resonant frequency of about 200 MHz. Reflections of RFpower at the stub-electrode interface are minimized or avoided becausethe resonant frequency of the stub 135 is set to be at or near theelectrode-plasma resonant frequency so that the two at least nearlyresonate together.

At the same time, reflections of RF power at the generator-stubinterface are minimized or avoided because the location of the tap 160along the axial length of the stub 135 is such that, at the tap 160, theratio of the standing wave voltage to the standing wave current in thestub 135 is near the output impedance of the generator 150 orcharacteristic impedance of the cable 162 (both being about 50 W). Howthe tap 160 is located to achieve this will now be discussed.

Axial Location of the Stub's Tap 160:

The axial length of the coaxial stub 135 preferably is a multiple of aquarter wavelength of a “stub” frequency (e.g., 220 MHz) which, asstated above, is near the electrode-plasma resonant frequency. In anexemplary embodiment, this multiple is two, so that the coaxial stublength is about a half wavelength of the “stub” frequency, or about 29inches.

The tap 160 is at a particular axial location along the length of thestub 135. At this location, the ratio between the amplitudes of thestanding wave voltage and the standing wave current of an RF signal atthe output frequency of the generator 150 corresponds to an inputimpedance matching the output impedance of the RF generator 150 (e.g.,50 Ohms). This is illustrated in FIGS. 2A and 2B, in which the voltageand current standing waves in the stub 135 have a null and a peak,respectively, at the shorted outer stub end 135 a. A desired locationfor the tap 160 is at a distance A inwardly from the shorted end, wherethe ratio of the standing wave voltage and current corresponds to 50Ohms. This location is readily found by the skilled worker byempirically determining where the standing wave ratio is 50 Ohms. Thedistance or location A of the tap 160 that provides a match to the RFgenerator output impedance (50 Ω) is a function of the characteristicimpedance of the stub 135, as will be described later in thisspecification. When the tap 160 is located precisely at the distance A,the impedance match space accommodates a 9:1 change in the real part ofthe load impedance, if the RF generator is of the typical kind that canmaintain constant delivered power over a 3:1 voltage standing wave ratio(VSWR).

In an exemplary embodiment, the impedance match space is greatlyexpanded to accommodate a nearly 60:1 change in the real part of theload impedance. This dramatic result is achieved by slightly shiftingthe tap 160 from the precise 50 W point at location A toward the shortedexternal end 135 a of the coaxial stub 135. This shift is, for example,5% of a wavelength in the exemplary embodiment (i.e., about 1.5 inch).It is a discovery of the reactor that at this slightly shifted taplocation, the RF current contribution at the tap 160 subtracts or addsto the current in the stub, which ever becomes appropriate, tocompensate for fluctuations in the plasma load impedance, as will bedescribed below with reference to FIGS. 3 and 4. This compensation issufficient to increase the match space from one that accommodates a 9:1swing in the real part of the load impedance to a 60:1 swing.

It is felt that this behavior is due to a tendency of the phase of thestanding wave current in the stub 135 to become more sensitive to animpedance mismatch with the electrode-plasma load impedance, as the tappoint is moved away from the “match” location at A. As described above,the electrode assembly 126 is matched to the negative capacitance of theplasma under nominal operating conditions. This capacitance is −50 to−400 pico farads at the preferred VHF source power frequency (210 MHz).At this capacitance the plasma exhibits a plasma impedance of (0.3+i7)Ω.Thus, 0.3Ω is the real part of the plasma impedance for which the systemis tuned. As plasma conditions fluctuate, the plasma capacitance andimpedance fluctuate away from their nominal values. As the plasmacapacitance fluctuates from that to which the electrode 125 was matched,the phase of the electrode-plasma resonance changes, which affects thephase of the current in the stub 135. As the phase of the stub'sstanding wave current thus shifts, the RF generator current supplied tothe tap 160 will either add to or subtract from the stub standing wavecurrent, depending upon the direction of the phase shift. Thedisplacement of the tap 160 from the 50 Ω location at A is limited to asmall fraction of the wavelength (e.g., 5%).

FIG. 3 illustrates the standing wave current in the stub 135 when thereal part of the plasma impedance has increased due to plasmafluctuations. In FIG. 3, the current standing wave amplitude is plottedas a function of axial location along the stub 135. A discontinuity inthe standing wave current amplitude at the location 0.1 on thehorizontal axis corresponds to the position of the tap 160. In the graphof FIG. 3, an impedance mismatch occurs because the real part of theplasma impedance is high, above the nominal plasma impedance for whichthe system is tuned (i.e., at which the electrode capacitance matchesthe negative plasma capacitance). In this case, the current at the tap160 subtracts from the standing wave current in the stub 135. Thissubtraction causes the discontinuity or null in the graph of FIG. 3, andreduces the delivered power to offset the increased load. This avoids acorresponding increase in delivered power (I²R), due to the higher load(R).

FIG. 4 illustrates the standing wave current in the stub 135 when thereal part of the plasma impedance decreases. In FIG. 4, the currentstanding wave amplitude is plotted as a function of axial location alongthe stub 135. A discontinuity in the standing wave current amplitude atthe location 0.1 marks the position of the tap 160. In the graph of FIG.4, the real part of the plasma impedance is low, below the nominalplasma impedance for which the system is tuned. In this case, thecurrent at the tap 160 adds to the standing wave current in the stub135. This addition increases the delivered power to offset the decreasedload, to avoid a concomitant decrease in delivered power, I²R, due tothe decreased load, R. With such compensation, much greater changes inload impedance can be accommodated so that the match space in increasedsignificantly.

This expansion of the match space to accommodate a 60:1 swing in thereal part of the load impedance enhances process window and reliabilityof the reactor. This is because as operating conditions shift during aparticular process or application, or as the reactor is operated withdifferent operating recipes for different applications, the plasmaimpedance will change, particularly the real part of the impedance. Inthe prior art, such a change could readily exceed the range of theconventional match circuit employed in the system, so that the deliveredpower could no longer be controlled sufficiently to support a viableprocess, and the process could fail. In the present reactor, the rangeof the real part of the load impedance over which delivered power can bemaintained at a desired level has been increased so much that changes inplasma impedance, which formerly would have led to a process failure,have little or no effect on a reactor embodying this aspect of theinvention. Thus, the invention enables the reactor to withstand fargreater changes in operating conditions during a particular process orapplication. Alternatively, it enables the reactor to be used in manydifferent applications involving a wider range of process conditions, asignificant advantage.

As a further advantage, the coaxial stub 135 that provides thisbroadened impedance match is a simple passive device with no “movingparts” such as a variable capacitor/servo or a variable frequency/servotypical of conventional impedance match apparatus. It is thusinexpensive and far more reliable than the impedance match apparatusthat it replaces.

De-Tuning the Operating and Resonant Frequencies to Broaden the ProcessWindow:

In accordance with a further aspect, the system Q is reduced to broadenthe process window by slightly offsetting the stub resonant frequency,the electrode plasma resonant frequency and the plasma source powerfrequency from one another. As described above, the stub resonantfrequency is that frequency at which the axial length of the stub 135 isa half wavelength, and the electrode-plasma resonant frequency is thefrequency at which the electrode assembly 126 and the plasma resonatetogether. In an exemplary embodiment, the stub 135 was cut to a lengthat which its resonant frequency was 220 MHz, the RF source powergenerator 150 was selected to operate at 210 MHz and the resultingelectrode-plasma resonant frequency was about 200 MHz.

By choosing three such differing frequencies for plasma resonance, stubresonance and source power frequency, rather than the same frequency forall three, the system has been somewhat “de-tuned”. It therefore has alower “Q”. The use of the higher VHF source power frequencyproportionately decreases the Q as well (in addition to facilitating thematch of the electrode and plasma capacitances under etch-favorableoperating conditions).

Decreasing system Q broadens the impedance match space of the system, sothat its performance is not as susceptible to changes in plasmaconditions or deviations from manufacturing tolerances. For example, theelectrode-plasma resonance may fluctuate due to fluctuations in plasmaconditions. With a smaller Q, the resonance between the stub 135 and theelectrode-plasma combination that is necessary for an impedance match(as described previously in this specification) changes less for a givenchange in the plasma-electrode resonance. As a result, fluctuations inplasma conditions have less effect on the impedance match. Specifically,a given deviation in plasma operating conditions produces a smallerincrease in VSWR at the output of RF generator 150. Thus, the reactormay be operated in a wider window of plasma process conditions(pressure, source power level, source power frequency, plasma density,etc). Moreover, manufacturing tolerances may be relaxed to save cost anda more uniform performance among reactors of the same model design isachieved, a significant advantage. A related advantage is that the samereactor may have a sufficiently wide process window to be useful foroperating different process recipes and different applications, such asconductor etch, dielectric etch and/or chemical vapor deposition.

Minimizing the Stub Characteristic Impedance to Broaden the ProcessWindow:

Another choice that broadens the tuning space or decreases the system Qis to decrease the characteristic impedance of the stub 135. However,the stub characteristic impedance preferably exceeds the generatoroutput impedance, to preserve adequate match space. Therefore, thesystem Q is preferably reduced, but only to the extent of reducing theamount by which the characteristic impedance of the stub 135 exceeds theoutput impedance of the signal generator 150.

The characteristic impedance of the coaxial stub 135 is a function ofthe radii of the inner and outer conductors 140, 145 and of thedielectric constant of the insulator 147 therebetween. The stubcharacteristic impedance is chosen to provide the requisite impedancetransformation between the output impedance of the plasma power source150 and the input impedance at the electrode 135. This characteristicimpedance lies between a minimum characteristic impedance and a maximumcharacteristic impedance. Changing the characteristic impedance of thestub 135 changes the waveforms of FIG. 2 and therefore changes thedesired location of the tap 160 (i.e., its displacement, A, from the farend of the stub 135). The allowable minimum characteristic impedance ofthe stub 135 is the one at which the distance A of FIG. 2 is zero sothat tap 160 would have to be located on the far end 135 a of thecoaxial stub 135 opposite the electrode 125 in order to see a 50 Ohmratio between the standing wave current and voltage. The allowablemaximum characteristic impedance of the stub 135 is the one at which thedistance A of FIG. 2 is equal to the length of the stub 135 so that thetap 160 would have to be close the near end 135 b of the coaxial stub135 adjacent the electrode 125 in order to see a 50 Ohm ratio betweenthe standing wave current and voltage.

In an initial embodiment, the coaxial stub characteristic impedance waschosen to be greater (by about 30%) than the output impedance of the RFgenerator 150, in order to provide an adequate match space. The stubimpedance must exceed the RF generator output impedance because theimpedance match condition is achieved by selecting the location of thetap point 160 to satisfyZ _(gen=a) ² [Z _(stub) ² /r _(plasma)]where a is determined by the location of the tap point and variesbetween zero and one. (a corresponds to the ratio of the inductance ofthe small portion of the stub 135 between the far end 135 b and the tap160 to the inductance of the entire stub 135.) Since a cannot exceedone, the stub characteristic impedance must exceed the generator outputimpedance in order to find a solution to the foregoing equation.However, since the Q of the system is directly proportional to the stubcharacteristic impedance, the amount by which the stub characteristicimpedance exceeds the generator output impedance preferably is somewhatminimized to keep the Q as low as practical. In the exemplaryembodiment, the stub characteristic impedance exceeds the generatoroutput impedance by only about 15 Ω.

However, in other embodiments, the coaxial stub characteristic impedancemay be chosen to be less than the plasma power source (generator) outputimpedance to achieve greater power efficiency with some reduction inimpedance match.

Increased Power Efficiency Provided by the Impedance Transformation ofthe Stub:

As discussed earlier in this specification, plasma operating conditions(e.g., plasma density) that favor plasma etch processes result in aplasma impedance that has a very small real (resistive) part (e.g., less0.3 Ohm) and a small imaginary (reactive) part (e.g., 7 Ohms).Capacitive losses predominate in the combination electrode-plasma areaof the system, because the electrode capacitance is the predominantimpedance to power flow in that part of the reactor. Therefore, powerloss in the electrode-plasma combination is proportional to the voltageon the electrode-plasma combination. In contrast, inductive andresistive losses predominate in the stub 135, because the inductance andresistance of the stub 135 are the predominant elements of impedance topower flow in the stub 135. Therefore, power loss in the stub 135 isproportional to current in the stub. The stub characteristic impedanceis much greater than the real part of the impedance presented by theelectrode-plasma combination. Therefore, in the higher impedance stub135 the voltage will be higher and the current lower than in the lowerimpedance plasma in which the current will be higher and the voltagelower. Thus, the impedance transformation between the stub 135 and theplasma-electrode combination produces a higher voltage and lower currentin the stub 135 (where resistive and inductive losses dominate and wherethese are now minimized) and a correspondingly lower voltage and highercurrent at the plasma/electrode (where capacitive losses dominate andwhere these are now minimized). In this manner overall power loss in thesystem is minimized so that power efficiency is greatly improved, asignificant advantage. In an exemplary embodiment, power efficiency isabout 95% or greater.

Thus, the stub 135, configured as described above, serves not only toprovide an impedance match or transformation between the generator andthe electrode-plasma impedances across a very wide range or window ofoperating conditions, but in addition provides a significant improvementin power efficiency.

Cross-Grounding:

The ion energy at the wafer surface can be controlled independently ofthe plasma density/overhead electrode power. Such independent control ofthe ion energy is achieved by applying an HF frequency bias power sourceto the wafer. This frequency, (typically 13.56 MHz) is significantlylower than the VHF power applied to the overhead electrode that governsplasma density. Bias power is applied to the wafer by a bias power HFsignal generator 200 coupled through a conventional impedance matchcircuit 210 to the wafer support 105. The power level of the biasgenerator 200 controls the ion energy near the wafer surface, and isgenerally a fraction of the power level of the plasma source powergenerator 150.

As referred to above, the coaxial stub 135 includes a shorting conductor165 at the outer stub end providing a short circuit between the innerand outer coaxial stub conductors 140, 145. The shorting conductor 165establishes the location of the VHF standing wave current peak and theVHF standing wave voltage null as in FIG. 2. However, the shortingconductor 165 does not short out the VHF applied power, because of thecoupling of the stub resonance and the plasma/electrode resonance, bothof which are at or near the VHF source power frequency. The conductor165 does appear as a direct short to ground for other frequencies,however, such as the HF bias power source (from the HF bias generator200) applied to the wafer. It also shorts out higher frequencies such asharmonics of the VHF source power frequency generated in the plasmasheath.

The combination of the wafer 110 and wafer support 105, the HF impedancematch circuit 210 and the HF bias power source 200 connected theretoprovides a very low impedance or near short to ground for the VHF powerapplied to the overhead electrode 125. As a result, the system iscross-grounded, the HF bias signal being returned to ground through theoverhead electrode 125 and the shorted coaxial stub 135, and the VHFpower signal on the overhead electrode 135 being returned to groundthrough a very low impedance path (for VHF) through the wafer, the HFbias impedance match 210 and the HF bias power generator 200.

The exposed portion of the chamber side wall between the plane of thewafer and the plane of the overhead electrode 125 plays little or norole as a direct return path for the VHF power applied to the overheadelectrode 125 because of the large area of the electrode 125 and therelatively short electrode-to-wafer gap. In fact, the side wall of thechamber may be isolated from the plasma using magnetic isolation or adielectric coating or an annular dielectric insert or removable liner.

In order to confine current flow of the VHF plasma source poweremanating from the overhead electrode 125 within the verticalelectrode-to-pedestal pathway and away from other parts of the chamber100 such as the sidewall, the effective ground or return electrode areain the plane of the wafer 110 is enlarged beyond the physical area ofthe wafer or wafer support 105, so that it exceeds the area of theoverhead electrode 125. This is achieved by the provision of the annularprocess kit 115, a conductive or semiconductive ring portion of whichmay be generally coplanar with and surrounding the wafer 110 andprovides a stray capacitance to the grounded chamber body. This extendsthe effective radius of the “return” electrode in the plane of the wafer110 for the VHF power signal from the overhead electrode. In anexemplary embodiment, the conductive or semiconductive ring portion ofthe process kit 115 is insulated from the grounded chamber body by adielectric ring portion 120 of the process kit 115. The thickness anddielectric constant of the dielectric ring 120 is selected to achieve adesirable ratio of VHF ground currents through the wafer 110 and throughthe conductive or semiconductive ring portion of the process kit 115.

In order to confine current flow from the HF plasma bias power from thebias generator 200 within the vertical path between the surface of thewafer and the electrode 125 and avoid current flow to other parts of thechamber (e.g., the sidewall), the overhead electrode 135 provides aneffective HF return electrode area significantly greater than the areaof the wafer or wafer support 105. The ring portion of the process kit115 in the plane of the wafer support 105 does not play a significantrole in coupling the HF bias power into the chamber, so that theeffective electrode area for coupling the HF bias power is essentiallyconfined to the area of the wafer and wafer support 105.

Enhancement of Plasma Stability:

Plasma stability was enhanced by eliminating D.C. coupling of the plasmato the shorting conductor 165 connected across the inner and outer stubconductors 140, 145 at the back of the stub 135. This is accomplished bythe provision of the thin capacitive ring 180 between the coaxial stubinner conductor 140 and the electrode 125. In the embodiment of FIG. 1,the ring 180 is sandwiched between the electrode 125 on the bottom andthe conductive annular inner housing support 176. In the exemplaryembodiments described herein, the capacitive ring 180 had a capacitanceof about 180 picoFarads, depending on the frequency of the bias chosen,about 13 MHz. With such a value of capacitance, the capacitive ring 180does not impede the cross-grounding feature described above. In thecross-grounding feature, the HF bias signal on the wafer pedestal isreturned to the RF return terminal of the HF bias generator 150 via thestub 135 while the VHF source power signal from the electrode 125 isreturned to the RF return terminal of the VHF source power generator 150via the wafer pedestal.

FIG. 5 is a graph illustrating the reflection coefficient between theVHF power source and the overhead electrode 125 as a function offrequency. This graph illustrates the existence of a very broad band offrequencies over which the reflection coefficient is below 6 dB, whichis indicative of the highly advantageous low system Q discussed above.

FIG. 6 illustrates the standing wave current as a function of positionalong the coaxial stub 135 in the case in which the tap 160 is placed atthe distance A of FIG. 2B from the shorted end of the stub.

FIG. 7 illustrates an alternative embodiment of the reactor in which theinner conductor 140 of the coaxial stub 135 is tapered, having a largerradius at the near stub end 135 b adjacent the overhead electrode 125and a smaller radius at the far stub end 135 a. This feature provides atransition between a low impedance (e.g., 50 W) presented by the coaxialstub 135 at the tap 160 and a higher impedance (e.g., 64 W) presented bythe coaxial stub 135 at the overhead electrode 125. Also, as shown inFIG. 7, the stub 135 need not be curved but instead can be straight.

As can be understood from the foregoing description, the inventivechamber concerns a capacitively coupled reactor having an overheadelectrode that is driven by a VHF plasma source power RF supply, ratherthan an HF power supply. We have found that at a VHF source powerfrequency, unlike an HF frequency, it is practical to tune the overheadelectrode to resonate with the plasma, leading to heretofore unattainedstability and efficiency and many other advantages.

The VHF capacitively coupled plasma reactor of the foregoing embodimentenjoys the advantage of very high etch selectivity and efficiency. Thegreat efficiency gives this reactor the ability to produce relativelyhigh density plasmas, rivaling those achieved in inductively coupledreactors. Yet, the VHF capacitively coupled reactor of the exhibits anetch selectivity that is far superior to that of an inductively coupledreactor. This is because the VHF capacitively coupled reactorexperiences far less residence time of process gas species and thereforeless dissociation of volatile species such as fluorine (relative toinductively coupled reactors).

Semiconductor device geometries are constantly being reduced by themicroelectronics industry in order to achieve higher device speeds. Suchan decrease in device size or geometry increases the aspect ratios ofcontact holes in the device structure, for example. As a result, etchprocesses must have correspondingly greater etch rates and etchselectivity. However, achieving a silicon dioxide high etch rate such as9,000 Angstroms per minute with a high silicon oxide-to-photoresist etchselectivity on the order of 10:1 seems to be impractical, even with ahighly efficient VHF capacitively coupled plasma reactor. This isbecause such performance would require a significant improvement in theplasma distribution uniformity over the wafer or workpiece surface thatsuch a reactor produces. Otherwise, as device geometries shrink, theprocess becomes more susceptible to failure due to overetching (inregions of higher plasma ion density) or premature etch stopping (inareas of lower plasma density). In order to avoid the overetchingproblem, the overall plasma density must be reduced, which reduces etchrate. The great need therefore is to find a way to improve the plasmaion distribution uniformity.

If somehow the plasma ion density distribution uniformity could beimproved in the VHF capacitively coupled reactor, then both excellentetch selectivity and high etch rate would be realized in the samereactor.

One type of reactor that has overcome the problem of non-uniform plasmadensity distribution is the magnetically enhanced reactive ion etch(MERIE) plasma reactor. An MERIE reactor is, typically, a capacitivelycoupled reactor in which HF frequency RF source power is applied to thewafer support pedestal and returned through the chamber ceiling or sidewalls. Its key feature is an array of electro-magnets producing a slowlycirculating magnetic field that circulates or stirs the plasma acrossthe plane of the workpiece. This stirring action provides a highlyuniform distribution of plasma density across the workpiece surface.However, an MERIE reactor does not enjoy the high plasma ion density andhigh etch rate of the VHF capacitively coupled reactor because it is notas efficient. Moreover, the MERIE reactor suffers from various problems:

-   -   (1) An MERIE reactor is prone to arcing between the wafer        support pedestal and metallic feed lines within the wafer        support pedestal. Such arcing diverts plasma source power from        plasma ion generation, and therefore impairs control of the        plasma ion density and therefore of the etch rate. In processes        in which the etch rate is critical because of extremely small        device geometries, such a loss of control may lead to process        failure.    -   (2) An MERIE reactor must employ such a powerful magnet array        (10-100 Gauss) that tends to promote device damage on the        workpiece. Such a powerful magnetic field is necessary in order        to produce a sufficient plasma density near the workpiece in        addition to circulating the plasma. Such a powerful magnetic        field is required to hold a significant portion of the plasma        against the plasma sheath adjacent the workpiece or wafer.

The main drawback, however, of an MERIE reactor is that it is incapableof attaining the high plasma density that the VHF capacitively coupledreactor readily provides. Thus, it would seem that reactors capable ofhigh plasma ion density and high etch selectivity (e.g., a VHFcapacitively coupled reactor) must be incapable of providing a highlyuniform plasma ion density distribution. Moreover, it would seem that areactor having good plasma ion density distribution uniformity (e.g., anMERIE reactor) must be incapable of producing high plasma ion density.

A superior way to feed process gases into a capacitively coupled plasmareactor is to inject the process gases through the overhead ceiling. Inthe present capacitively coupled plasma reactor, the overhead ceiling isthe source power electrode that is coupled to a VHF RF power generatorthrough a coaxial stub or equivalent impedance match element. In orderto inject the process gas from the ceiling, the ceiling electrode alsois a “showerhead”, a conductive layer having a set of small gasinjection ports passing through it for injecting the process gases. Insome instances plasma discharge or “arcing” tends to occur within theceiling gas injection ports. This poses a risk of the plasma dischargeremoving material of the overhead electrode or showerhead from theinterior surfaces of the gas injection ports. The species (e.g.,metallic species) thus introduced into the plasma can contaminate thesurface of the wafer or workpiece and damage the microelectronic devicesbeing fabricated thereon.

Accordingly, it would be desirable to reduce or eliminate the tendencyto ignite plasma within gas injection ports (or anywhere else outside ofthe bulk plasma),particularly in a plasma reactor having a combinationoverhead electrode/gas distribution showerhead connected to a VHF plasmasource RF power supply.

The overhead electrode may suffer wear from being in contact withplasma, particularly since it is both an anode for the bias power and acathode for the source power and is therefore subject to RF and DCcurrents. The cost of operating the reactor would be reduced if therecould be found a way to avoid passing such currents directly through theoverhead electrode or avoid direct contact of plasma with the electrode.

A problem generally found in plasma reactors is the generation of secondand third harmonics within the plasma sheath. In the present reactor,while plasma VHF source power is applied by the overhead electrode,plasma bias power is applied by an HF signal on the wafer supportpedestal. At HF frequencies most of the RF power is consumed in thesheath, the remainder sustaining the bulk plasma. A plasma sheath is anon-linear load and therefore creates second and/or third harmonics ofthe HF bias signal applied to the wafer support pedestal. The presenceof such harmonics changes plasma behavior in such a way that processperformance is impaired in the presence of such harmonics. Specifically,process control to avoid etch stop or over-etching becomes moredifficult, and the etch rate is reduced. It would be desireable toreduce the production of such harmonics affecting the plasma

In the present reactor, the coaxial tuning stub, whose length iscorrelated to the wavelength of the VHF source power signal, can have afootprint larger than the remainder of the reactor. It would thereforebe advantageous to be able to reduce this footprint without sacrificingany of the advantages of the coaxial tuning stub.

It is a discovery of the present invention that combining certainfeatures of the MERIE reactor with the VHF capacitively coupled reactorof FIGS. 1-7 solves the problems enumerated above for each of them, andthat such a combination enjoys all the advantages and none of thedisadvantages of the two types of reactors. A reactor in accordance withthis combination is realized by adding an array of MERIE magnets to theVHF capacitively coupled reactor of FIGS. 1-7. The circulation of theplasma by the MERIE magnets solves the problem of plasma iondistribution non-uniformity in the VHF reactor. However, an MERIEreactor requires the process gases to be fed from an overhead gasdistribution plate or showerhead. As will be discussed in detail belowin this specification, providing such a gas distribution plate in theVHF capacitively coupled reactor of FIGS. 1-7 is fraught with problemsdue to the high plasma density of such a reactor. Specifically, such areactor is susceptible to arcing within the gas injection ports in thegas distribution plate. In order to achieve the desired combination ofthe VHF capacitively coupled reactor with an MERIE reactor, the problemof arcing within the gas injection ports had to be solved, and thissolution is described in a later section of this specification.

One surprising result of this combination is that the higher efficiencyof the VHF reactor of FIGS. 1-7 produces such a high plasma density thatthe magnetic field of the MERIE magnets may be reduced by a factor twoor more. Specifically, the MERIE magnetic field must typically lie in arange of about 10-100 Gauss. However, in the VHF capacitively coupledreactor, because of the much greater plasma ion density, an MERIEmagnetic field can achieve the same plasma distribution uniformity withonly half the field, about 30-60 Gauss. This is because the MERIEmagnetic field needs to do less work to draw the same amount of plasmanear the plasma sheath over the wafer which is stirred by thecirculation of the MERIE magnetic field. Thus, the lesser magnetic fieldcan provide the same optimum uniformity of plasma ion densitydistribution. The reduction in the magnetic field decreases the amountof device damage caused by plasma ions. The reduction by 50% in theMERIE magnetic field not only reduces device damage but essentiallyeliminates it. This solves the problem of device damage to which typicalMERIE reactors are prone.

Another surprising result is that the presence of VHF source powerprevents arcing at the wafer support pedestal that plagues typical MERIEreactors, as referred to above. This in turn greatly improves theprocess control in an MERIE reactor. Such arcing was prone to occurbetween the wafer support pedestal and metal gas or coolant feed lineswithin or under the wafer support pedestal.

In exemplary embodiments, the VHF source power frequency is well-abovethe cyclotron resonance frequency associated with the MERIE magnets.This prevents the formation of electron cyclotron resonance of plasmaelectrons with the field of the MERIE magnets, which would divert VHFsource power from ion generation, and thereby inhibit control over theplasma ion density. The VHF source power frequency is selected asdescribed above with reference to FIGS. 1-7 so that the plasma frequencyis at or near the resonance frequency of the overhead electrode. Thisfrequency is well-above the cyclotron resonance frequency associatedwith the field of the MERIE magnets (e.g., 30 Gauss). The electroncyclotron frequency is defined as:qB/(m _(e)2Π)where q is the charge of an electron, B is the magneic field of theMERIE magnet array, and m_(e)is the electron mass. This equation showsthat the cyclotron frequency is proportional to the magnetic field, andthis points to the advantage realized by the use of VHF source power: Asreferred to above, the VHF capacitively coupled reactor of FIGS. 1-7 iscapable of generating such a much higher plasma density than aconventional MERIE reactor that the MERIE magnetic field may bedramatically reduced when used in the VHF reactor. This not only resultsin a great decrease in device damage, as referred to above, but inaddition reduces the electron cylcotron resonance frequency well belowthe VHF source power frequency. In one embodiment, the cyclotronresonance frequency associated with the MERIE magnets was computed to beabout 150 MHz and the VHF source power frequency that resulted in nearmatching of the plasma resonant frequency with the overhead electroderesonant frequency was about 176 MHz. In other embodiments the VHFsource power frequency is over 200 MHz, which is even further above thecyclotron resonance frequency.

The combination of the VHF capacitively coupled plasma reactor of FIGS.1-7 with MERIE magnets results in a plasma reactor having threeindependently controllable parameters:

-   -   (1) the plasma ion density, controlled by the VHF source power        level,    -   (2) the ion energy at the wafer surface, controlled by the HF        bias power level applied to the wafer support pedestal,    -   (3) the degree of plasma ion uniformity distribution, controlled        by the magnetic field of the MERIE magnets.

Such a reactor has outstanding performance. Specifically, the reactorcan couple VHF source power of 4000 Watts to the plasma with about 95%efficiency for VHF frequencies up to 250 MHz, and for chamber pressureswithin a range of about 1 mT to 3000 mT.

In such an MERIE VHF reactor, the process gases are best introducedthrough the overhead VHF source power electrode. For this purpose, theoverhead electrode is endowed with the function of a gas distributionshowerhead, by providing an array of small gas injection nozzles orports through the overhead electrode. The process gases are fed to theseinjection ports through the center conductor of the coaxial tuning stub.Since the center conductor is coupled to the overhead electrode, theprocess gas feeds are completely protected from the plasma and fromelectric fields.

Arcing and other potential issues are avoided while retaining all of theabove-described advantages through any of a combination of features, oneof which is to put the overhead electrode at a floating D.C. potentialby capacitively isolating it from the VHF tuning stub. This isaccomplished by placing a dielectric film between the coaxial tuningstub and the overhead electrode. This feature prevents DC plasma currentfrom returning through the tuning stub via the overhead electrode, andthereby reduces arcing within the gas injection holes in the overheadelectrode.

Another feature that reduces arcing is to provide capacitance betweenthe plasma and the overhead electrode. For this purpose a dielectriclayer is formed on the electrode surfaces of the overhead electrode thatface the plasma. Preferably, this is done by anodizing such electrodesurfaces, particularly the interior surfaces of the gas injection portsin the electrode. This feature helps to obviate plasma arcing in the gasinjection ports in the overhead electrode. One reason for this is thatthe capacitance of the anodized electrode surfaces provides chargestorage capacity which permits some charge of the RF current from theplasma to be stored rather than passing on to the electrode surfaces. Tothe extent charge is thus diverted from the surfaces of the gas inletports in the overhead electrode, plasma ignition therein is avoided.

In addition to avoiding plasma arcing within the gas injection ports ofthe overhead electrode, the feature of capacitively isolating theoverhead electrode extends the useable life of the electrode because itresults in no net D.C. current between the plasma and the electrode, asignificant advantage.

In order to further reduce the risk of plasma arcing in the gasinjection ports, another feature is introduced, namely a metal orceramic “foam” layer between the coaxial stub and the capacitive layerlying between the electrode and the coaxial tuning stub. In oneembodiment, the metal foam layer is of a diameter that is generallycoextensive with the overhead electrode. The metal foam layer is of thecommercially available type well-known in the art and typically consistsof an aluminum matrix having a random cell structure. The advantage ofthe metal foam layer is that it suppresses electric fields near theelectrode (i.e., within a plenum above the overhead electrode) andthereby reduces the tendency of plasma to arc inside the gas injectionports in the overhead electrode.

A metal foam layer is also employed to baffle the incoming process gasin order to achieve an even gas distribution across the array of gasinjection ports in the overhead electrode. Preferably, the gas injectionholes or ports in the overhead ceiling are divided into a radially innergroup and a radially outer group. One metal foam layer baffles gasbetween a first gas supply and the outer group of ports, while anothermetal foam layer baffles gas between a second gas supply and the innergroup of ports. The radial distribution of process gas flow may beadjusted by independently adjusting the gas flow rates of the two gassupplies.

As described in the above-referenced parent application, the coaxialtuning stub and overhead electrode offer a low impedance RF return pathto ground for the HF bias power applied to the wafer support pedestal.However, it has been discovered that the new capacitive dielectric layernow inserted between the coaxial tuning stub and the overhead electrodecan be used to tune the return HF path through the overhead electrode toa particular HF frequency. One advantage of the choice of a VHF sourcepower frequency (on the overhead electrode) is that the capacitive layer(between the overhead electrode and the tuning stub), if tuned for HFfrequencies, does not affect the VHF signal applied to the overheadelectrode because it is an electrical short for a broad band of VHFfrequencies.

Initially, a narrow HF frequency pass band to which the RF return pathis tuned by the added capacitive layer was centered at the frequency ofthe HF bias source power applied to the wafer support pedestal. However,the problem of sheath-generated harmonics can be solved by insteadselecting this capacitance to tune the HF return path through theoverhead electrode to the second harmonic of the HF bias power signal.The result of this selection is that the HF second harmonic generated inthe plasma sheath near the overhead electrode is shunted to groundthrough the overhead electrode before it can significantly affect thebulk plasma. The etch rate was found to be improved by this feature by10% to 15% in one embodiment. In this case, it is believed thefundamental of the HF bias signal is returned to ground through otheravailable RF paths, such as the chamber side wall.

As will be described below in detail, the selection of the capacitanceof this added capacitive layer (between the overhead electrode and thetuning stub) for resonance at the selected HF frequency must take intoaccount not only the capacitance of the thin plasma sheath at theoverhead electrode but also the capacitance of the thick plasma sheathat the wafer support pedestal.

The highly efficient VHF plasma source of the present invention iscapable of maintaining a plasma of sufficiently high density so that itmay be used to thoroughly dry-clean the chamber interior periodically.As employed in this specification, the term “dry-clean” refers to acleaning procedure requiring no application of liquid chemical agentsbut only the application of a plasma, so that the vacuum enclosure neednot be opened. Since in this manner the chamber can be thoroughlycleaned of polymer residue, its surfaces during wafer processing may bekept at a sufficiently high temperature to continually evaporate anypolymer deposits thereon, so that the chamber is kept at least nearlyfree of polymer deposits throughout processing. (In contrast, for areactor that cannot be thoroughly cleaned, plasma conditions must becontrolled so that polymer deposits on chamber wall surfaces continue toadhere rather than being removed, to avoid contamination of theprocess.) For this purpose, the overhead electrode assembly includesliquid passages for introducing fluid for heating or cooling theoverhead electrode, enabling temperature control of the externalsurfaces thereof. Preferably, the plasma conditions (ion energy, walltemperatures, etc.) are such that no polymer accumulates on the chambersurfaces during processing. Any minor accumulations are thoroughlyremoved during cleaning.

One advantage of such a feature is that an optical window may beprovided on or adjacent the overhead electrode, because it will remainclear or free of polymer deposits during processing. Thus, the reactorperformance may be optically monitored. Accordingly, the overheadelectrode optionally includes an optical window near its center, with alight transmitting optical fiber cable extending upwardly for connectionto sensors outside of the chamber. The optical monitoring of the plasmaprocess may be employed to perform endpoint detection. For example, theoptical monitor may measure decreasing layer thickness in a plasma etchprocess or increasing layer thickness in a plasma-assisted chemicalvapor deposition process, using conventional optical measurementtechniques.

In order to solve the problem of contamination from material of theexposed surfaces of the overhead electrode entering the plasma andeventually reaching the wafer or workpiece, an additional outer layer isintroduced onto the bottom (plasma-facing) surface of the overheadelectrode. This additional outer layer is formed of a materialcompatible with the particular process being carried out. For example,in a silicon dioxide etch process, the outer layer on the overheadelectrode would be silicon or silicon carbide. Optionally, prior to theplacement of this outer layer, the overhead electrode plasma-facingsurface is anodized, as mentioned hereinabove.

Another discovery of the present invention is that the plasma canexhibit a greater resistive load impedance variation and a smallerreactive load impedance variation than was earlier expected.Specifically, the resistive load impedance may vary by as much as 100:1(instead of 60:1) while the reactive load impedance may vary by only 20%(instead of 35%). This difference enables the characteristic impedanceof the coaxial tuning stub to be reduced from 65 Ohms (above the RFgenerator's 50 Ohm output impedance) down to 30 Ohms (below the RFgenerator's output impedance). This reduction achieves a proportionalincrease in tuning space with a very small compromise in efficiency.Specifically, the range of variations in plasma resistive load impedancewhich can be matched by the tuning stub is increased from 60:1 (as inthe parent application) to 100:1, due to the reduction in coaxial stubcharacteristic impedance. The characteristic impedance of the coaxialstub is determined by the radii of its inner and outer conductors, asset forth in the above-referenced parent application.

In order to reduce the footprint of the coaxial tuning stub, anequivalent strip line circuit is substituted in its stead. The outerconductor of the coaxial tuning stub becomes a ground plane surface asthe metal lid capping the reactor, while the center conductor of thecoaxial tuning stub becomes the strip line conductor. The characteristicimpedance of the strip line conductor is adjusted by adjusting thespacing between the strip line conductor and the ground plane (the lid).The footprint of the tuning device is reduced because, while the coaxialtuning stub extends along a straight line, the strip line conductor canwind around circularly inside the lid, thereby reducing the area orfootprint. All of the features of the coaxial tuning stub are retainedin the strip line circuit. Thus, the length of the strip line conductoris determined in the same manner as the length of the coaxial tuningstub as described above. Also, the location along the length of thestrip line conductor for the feed point or tap connected to the VHFgenerator is the same as that of the tap to the coaxial tuning stub asdescribed in the above-referenced parent application. Also, the stripline conductor is hollow and utilities are fed through the strip lineconductor, in the same manner that utilities are fed through the coaxialtuning stub center conductor as described in the above-referenced parentapplication.

Structure of the VHF Capacitively Coupled Reactor with MERIE Magnets:

Referring to FIGS. 8 and 9, a VHF capacitively coupled plasma reactorincludes the following elements found in the reactor of FIG. 1: areactor chamber 100 with a wafer support 105 at the bottom of thechamber supporting a semiconductor wafer 110. A process kit in theillustrated embodiment consists of a semi-conductive or conductive ring115 supported by a dielectric ring 120 on the grounded chamber body 127.The chamber 100 is bounded at the top by a disc shaped overhead aluminumelectrode 125 supported at a predetermined gap length above the wafer110 on grounded chamber body 127 by a dielectric seal 130. The overheadelectrode 125 also may be a metal (e.g., aluminum) which may be coveredwith a semi-metal material (e.g., Si or SiC) on its interior surface, orit may be itself a semi-metal material. An RF generator 150 applies RFpower to the electrode 125. RF power from the generator 150 is coupledthrough a coaxial cable 162 matched to the generator 150 and into acoaxial stub 135 connected to the electrode 125. The stub 135 has acharacteristic impedance, resonance frequency, and provides an impedancematch between the electrode 125 and the coaxial cable 162/RF powergenerator 150, as will be more fully described below. The chamber bodyis connected to the RF return (RF ground) of the RF generator 150. TheRF path from the overhead electrode 125 to RF ground is affected by thecapacitance of the process kit dielectric ring 120 and the dielectricseal 130 . The wafer support 105, the wafer 110 and the process kitsemiconductive (or conductive) ring 115 provide the primary RF returnpath for RF power applied to the electrode 125.

The improvement in plasma density distribution uniformity is achieved bythe introduction of a set of MERIE electromagnets 901, 903, 905, 907spaced equally about the periphery of the wafer support pedestal andoutside of the reactor chamber. These MERIE magnets are adapted toproduce a magnetic field that slowly rotates about the axis of symmetryof the cylindrical chamber generally across the surface of the wafersupport pedestal. In one embodiment this feature is realized by theMERIE magnets 901, 903, 905, 907 having electromagnet windings woundabout respective axes tangent to the circumference of the wafer supportpedestal. In this one embodiments, an MERIE current controller 910controls the individual current to each MERIE magnet. A circulatingmagnetic field is generated in the plane of the workpiece support by thecontroller 910 providing individual AC currents to each of theindividual magnet windings of the same frequency but offset in phase by90 degrees (or by 360 degrees divided by the number of MERIE magnets).In an alternative embodiment, the feature of a rotating magnetic fieldis realized by a support frame 1020 (dashed line) supporting all of theMERIE magnets that is rotated about the axis of symmetry by a rotor 1025(dashed line). In this alternative embodiment, the MERIE magnets arepermanent magnets.

A second array of MERIE magnets 912, 914, 916, 918 equally spaced aboutthe workpiece or wafer support pedestal but in a higher plane than thefirst set of MERIE magnets 901, 903, 905, 907 may be provided as well.Both sets of magnets lie in respective planes that are near the plane ofthe workpiece support.

The controller 910 applies a low frequency (0.5-10 Hz) AC current toeach of the electromagnets 901-907, the phases of the currents appliedto neighboring magnets being offset as described above by 90 degrees.The result is a magnetic field that rotates about the axis of symmetryof the workpiece support at the low frequency of the AC current. Themagnetic field causes the plasma to be drawn toward the magnetic fieldnear the workpiece surface and to circulate with the field. This stirsthe plasma so that its density distribution becomes more uniform. As aresult, reactor performance is significantly improved because moreuniform etch results are obtained across the entire surface of thewafer.

Introduction of Process Gas Through the Overhead Electrode:

As referred to previously herein, an MERIE reactor is best implementedby feeding the processes gas from the overhead ceiling. In the presentinvention, this requires feeding the process gases through the overheadelectrode 125. For this purpose, the overhead electrode 125 in theembodiment of FIGS. 8 and 9 is a gas distribution showerhead, andtherefore has a large number of gas injection ports or small holes 300in its bottom surface 125 a facing the workpiece support 105. In anexemplary embodiment, the ports 300 were between 0.01 and 0.03 inch indiameter and their centers are uniformly spaced apart by about ⅜ inch.In the embodiment illustrated in FIG. 8, the annular top 290 a of aconical metal housing 290 supports the near end 140 a of the coaxialstub inner conductor 140 and its annular base 290 b rests on thealuminum overhead electrode 125. The conical shape of the housing 290defines a large open plenum over the overhead electrode 125 within whichvarious utilities may be fed from the hollow coaxial inner conductor 140to the overhead electrode 125. As will be described in more detailbelow, the conical housing base 290 b is near the outer circumference ofthe overhead electrode 125, leaving nearly all of the upper surface ofthe overhead electrode 125 accessible.

In this embodiment, the ports 300 consist of a radially outer group of0.020 in diameter ports 302 and a radially inner group of 0.010 indiameter ports 304. The outer group of ports 302 may extends beyond thecircumference of the wafer, in order to ensure uniform gas flow at thewafer periphery. One advantage of this feature is that the radialdistribution of process gas flow can be adjusted in such a manner as tocompensate for the tendency of the VHF capacitively coupled reactor ofFIGS. 1-7 to produce a plasma density that is greater over the center ofthe wafer and less over the wafer periphery.

A radially outer aluminum foam layer 310 within the overhead electrode125 overlies the ports 302. A radially outer gas distribution manifoldor plenum 315 overlying the outer foam layer 310 is coupled through anaxial gas passageway 320 to a gas supply line 325 passing through theinterior conductor 140 of the coaxial tuning stub 135. A radially inneraluminum foam layer 330 within the overhead electrode 125 overlies theports 304. A radially inner gas distribution manifold or plenum 335overlying the inner foam layer 330 is coupled through an axial gaspassageway 340 to a gas supply line 345 passing through the interiorconductor 140 of the coaxial tuning stub 135. The aluminum foam layers310 and 330 baffle the incoming process gases. The radial distributionof process gas flow rate is adjusted by independent selection of processgas flow rates within each one of the gas supply lines 325 and 345.

Suppression of Arcing in the Gas Injection Ports:

In order to provide some capacitance between the plasma and the overheadelectrode as a means of reducing arcing, the overhead electrode bottomsurface 125 a is coated with a dielectric layer. For example, theoverhead electrode 125 is aluminum and the dielectric coating is formedby anodizing the electrode bottom surface 125 a. Such anodization formsa very thin dielectric coating not only on the flat bottom surface 125 abut also on the interior surfaces of the gas injection ports 300. Thisfeature tends to suppress arcing within the gas injection ports byproviding a charge storage capability that can compensate for RF plasmacurrents flowing to the overhead electrode 125. FIG. 10 is an enlargedpartial view corresponding to FIG. 8 illustrating the resulting finestructure near one of the gas inlet ports 300. In particular, analuminum oxide layer 350 formed by anodization covers the electrodebottom surface 125 a and covers the interior surface of the gasinjection port 300.

In order to suppress electric fields near the overhead electrode 125,the top surface 125 b of the overhead electrode 125 is covered with arelatively thick (0.25 in) layer of aluminum foam 355. The thickaluminum foam 355 tends to keep the electric potential near the overheadelectrode constant in the axial (vertical) direction, therebysuppressing electric fields in that vicinity which would otherwisecontribute to plasma arcing within the gas injection ports 300.

In order to block D.C. plasma currents from flowing through the overheadelectrode to the coaxial stub center conductor 140, a thin insulativelayer 360 is placed between the overhead electrode 125 and the base 290b of the conductive housing 290 that connects the overhead electrode 125to the coaxial center conductor 140. This feature allows the D.C.potential of the overhead electrode to float. A capacitor is therebyformed between the overhead electrode 125 and the conductive housingbase 290 b. The capacitance of this capacitor is determined by the areaof the base 290 b as well as by the thickness and dielectric constant ofthe thin insulative layer 360. Preferably, the capacitance of thiscapacitor is selected to provide a narrow resonance or low impedancepath at a particular HF frequency, while providing an RF short acrossthe entire VHF band. In this way, the overhead electrode 125 provides areturn path for HF bias power applied to the wafer support pedestal 105,but does not affect the behavior of the overhead electrode 125 at theVHF source power frequency. By thus, blocking D.C. plasma current thatwould otherwise flow to the overhead electrode, plasma arcing within thegas injection ports 300 is suppressed because such D.C. currents wouldcontribute to arcing.

In summary, plasma arcing within the gas injection ports 300 issuppressed by one or more of the following features: (a) forming adielectric coating 350 on the bottom of the overhead electrode 125 andon the interior surfaces of the gas injection ports 300, (b) providing ametallic aluminum foam layer 355 on top of the overhead electrode 125,and (c) placing a thin insulative layer 360 between the overheadelectrode 125 and the conductive housing 290.

Suppression of Plasma Sheath-Generated Harmonics:

The thin insulative layer 360 plays a role in suppressing plasmasheath-generated harmonics of the HF bias signal applied to the wafersupport pedestal 105. The presence of such harmonics degrades processperformance, and specifically reduces etch rates. By selecting thecapacitance-determining characteristics of the insulative layer 360(i.e., dielectric constant and thickness), the return path from theplasma through the overhead electrode 125 and coaxial inner conductor140 is tuned to resonate (and therefore have a very high admittance) ata particular HF frequency. While one choice for this resonant frequencywould be the fundamental of the HF bias signal applied to the wafersupport pedestal 105, it is a discovery of the invention that the etchrate is improved by 10% to 15% by selecting this resonance to be thesecond harmonic of the bias signal. Such a favorable result is achievedbecause harmonics generated by the non-linear load presented by theplasma sheath are quickly returned to ground through the low impedancepath presented by the overhead electrode and coaxial center conductor140 by virtue of the capacitive layer 360.

Selection of the thickness of the capacitor layer 360 to tune the returnpath through the overhead electrode 125 to a particular HF frequency isaffected by a number of factors, including the capacitance of the thinplasma sheath at the overhead electrode 125, the capacitance of thethick plasma sheath at the wafer support pedestal 105 as well as thecapacitance of the plasma itself. Numerous conventional techniques maybe readily employed by the skilled worker to find the correct thicknessof the capacitor layer 360 to achieve resonance at the selected HFfrequency given the particular plasma operating conditions, includingtrial and error.

Electrode Surface Temperature Control:

In an oxide etch reactor, polymer deposits are a serious problem becausethe process gas must be able to form polymer layers over non-oxidecontaining surfaces on the workpiece in order to achieve a suitable etchselectivity between silicon dioxide materials and other materials thatare not to be etched. During plasma processing using flourocarbon gases,the simpler fluorine ions and radicals perform the etching while thecarbon-rich species deposit polymer over all non-oxygen-containingmaterials on the workpiece as well as all interior surfaces of thereactor chamber. In order to avoid contamination of the workpiece bypolymer particles falling from chamber interior surfaces into theplasma, these surfaces must be kept at a sufficiently low temperatureand the plasma electron energy must be kept sufficiently low to avoidtearing such deposits off of the chamber interior surfaces.Alternatively, the chamber vacuum must be interrupted and a chemicalcleaning step performed to remove such deposits, a step that greatlyreduces productivity of the reactor.

The capacitively coupled VHF source described with reference to FIG. 1is highly efficient and therefore capable of producing, during anon-chemical cleaning step, a sufficiently high plasma density tothoroughly remove from the chamber interior surfaces any polymer residuedeposited during wafer processing. During such a cleaning step, theusual plasma process gases may be replaced by a more volatile gas (e.g.,one tending to produce a plasma with a very high free fluorine content).Since no liquid chemicals need be introduced into the chamber, thechamber remains closed so that the cleaning step may be performedquickly and frequently to keep the chamber free of polymer deposits.Therefore, an operating mode of the reactor of FIG. 8 is one in whichthe chamber surface temperatures and the plasma ion energies aresufficiently great to avoid accumulation of polymer on the interiorchamber surfaces.

For this purpose, the reactor of FIG. 8 includes passages 670 (forheat-conducting fluid) on the overhead electrode 125. In theimplementation of FIG. 8, the fluid passages 670 are formed between theupper aluminum foam layer 355 and the upper surface of the overheadelectrode 125. Alternatively, such passages may be formed completelyinternally within the overhead electrode 125. A temperature-controllingfluid or gas is fed to the fluid passages 670 from a fluid supply line675 passing through the hollow inner coaxial conductor 140. Thus, thetemperature of the overhead electrode 125 may be precisely controlled.By thus controlling the electrode temperature and by controlling otherplasma process parameters such plasma ion energy, the reactor may beoperated in either deposition mode (in which the surfaces aresufficiently cool to accumulate polymer) or in a depletion mode (inwhich the surfaces are sufficiently hot to allow plasma ions to tearaway polymer from the surfaces and thereby avoid accumulation ofpolymer). The depletion mode is effective because this mode betteravoids particle contamination.

Optical Monitoring of the Plasma Process:

Since the reactor of FIG. 8 can be operated so as to be free of polymerdeposits on the chamber interior surfaces, an optical window 680 may beprovided in the bottom surface of the overhead electrode 125. An opticalchannel such as an optical fiber or light pipe 685 is connected at oneend to the optical window 680 and passes through the hollow innercoaxial conductor 140. The light pipe 685 is connected to a conventionoptical detector 687 at the outer end.

With this feature, end point detection and other measurements may beperformed using such an optical detector. Specifically, the detector 687measures the thickness of a selected layer on the workpiece orsemiconductor wafer 110, using well-known optical techniques. During anetch process, for example, the process would be halted after thethickness of the material being etched is reduced to a predeterminedthickness, as measured by the detector 687.

Prevention of Contamination:

Since the chamber interior surfaces can be maintained free of polymerdeposits, they remain exposed to the plasma. In particular, the bottomsurface of the aluminum overhead electrode 125 is continually subject toattack from the plasma, and is therefore liable to contribute aluminumspecies into the plasma, leading to contamination of the workpiece andhence process failure. In order to prevent such a problem, the bottomsurface of the overhead electrode 125, which may be anodized, is coatedwith a process-compatible material such as silicon or silicon carbide.Thus, as shown in FIGS. 10 and 11A, a thin silicon carbide film 690covers the bottom anodized surface of the aluminum overhead electrode125. The thin silicon or silicon carbide film 690 prevents the plasmafrom attacking the aluminum material of the electrode 125. To the extendthe plasma removes material from the silicon-containing film 690, thespecies thus introduced into the plasma cannot contaminate the processbecause such species (silicon and carbon) are already present in theplasma and/or workpiece and therefore are compatible with the process.Silicon is present in the plasma where silicon oxide is being etched.Carbon is in the plasma wherein fluoro-carbon gases are employed asprocess etch gases.

In an alternative embodiment, the overhead electrode is not anodized andthe silicon carbide film 690 is formed over a pure aluminum surface ofthe electrode 125. In another alternative embodiment illustrated in FIG.11B, the gas injection holes 300 are L-shaped (involving two right-angleturns) and their openings into the chamber are annular, the annularopening of each hole 300 being defined by a solid disk 300 a blockingthe center of each opening.

Results:

The invention thus provides a plasma reactor which is far less sensitiveto changes in operating conditions and/or variations in manufacturingtolerances. It is believed that these great advantages including lack ofsensitivity to operating conditions—i.e., broad tuning or frequencyspace for impedance matching—are the contributions of at least one ormore of a number of reactor features. These features include an overheadreactor electrode having a capacitance matching or nearly matching themagnitude of the negative capacitance of the plasma at the most desiredprocessing plasma ion densities, use of a VHF source power frequencymatching or nearly matching the plasma-electrode resonance frequency;the close relationship of the VHF source power frequency, theplasma-electrode resonance frequency and the stub resonance frequency;offsetting the plasma-electrode resonance frequency, the stub resonancefrequency and the source power frequency from one another; and the useof a resonant stub match to couple source power to the overheadelectrode, preferably with the source power input tap 160 offsetslightly from the ideal match location.

It is believed that offsetting the plasma, stub and source powerfrequencies broadens the tuning space of the system by, in effect,de-tuning the system. Using a stub match broadens the tuning space bymatching across a broader frequency range. Offsetting the stub tap point160 from the ideal match point further optimizes the system to broadenthe tuning space, because this feature has the effect of adding currentwhen delivered power would otherwise decline and of subtracting currentwhen delivered power would otherwise increase. Using a higher (VHF)source power frequency provides a decrease in system Q or an increase intuning space proportional to the increase in source power frequency.More importantly, this selection allows the electrode-plasma resonanceto be matched to the source power frequency at a plasma densityfavorable to etch processes.

Because the invention renders the reactor virtually immune to changes inprocess conditions over a broader process window, it provides thethree-fold advantage of a reactor that is (a) workable over a widerrange of process condition deviations, (b) useful over a broader rangeof applications (different process recipes) and (c) whose performance isvirtually unaffected over a wider range of manufacturing tolerances, sothat reactor-to-reactor characteristics are uniform.

Consequently, superior results have been attained. Specifically, the Qof the system has been minimized to about 5 in some cases to retain asuperior degree of uniformity of characteristics and performance amongdifferent reactors of the same model, and to enhance process window.High plasma densities on the order of 10¹² ions/cc have been achievedconsistently with only 2 kW of source power. The system sustainedplasmas over a pressure range of 10 mT to 2.00 mT with no transitionswith source power levels as low as 10 W. The shorted impedance matchingcoaxial stub resonating near the VHF plasma and source power frequenciesshorted out parasitic VHF plasma sheath harmonics while realizing apower efficiency in excess of 95%. The system accommodated plasmaresistive load variations of 60:1 and reactive load variations of 1.3 to0.75 while maintaining the source power SWR at less than 3:1.

It is believed that this increased capability to accommodate loadvariations, and hence expanded process windows, is due in large part to(a) the matching of the electrode and plasma capacitances under thedesign operating conditions, accomplished as above described byappropriate choice of dielectric values between the electrode 125 andits conductive support as well as the appropriate choice of VHF sourcepower frequency; and (b) the specially configured coaxial stub with theoptimal tap positioning, by which the tap current added to the stubcurrent under low load conditions and subtracted from it under high loadconditions. It is believed the very high power efficiency is due inlarge part to the impedance transformation provided by the coaxial stub,which minimizes reflection losses both at the generator connection aswell as at the electrode connection, due to obtaining a match betweenstub resonant frequency and electrode-plasma resonant frequency, alongwith optimal tap positioning for realizing a low current and highvoltage in the coaxial stub where resistive losses dominate and a highcurrent low voltage at the electrode/plasma where capacitive lossesdominate. Yet all these benefits are provided while avoiding orminimizing the need for conventional impedance match apparatus.

While exemplary embodiments of the invention adapted for dielectric andconductor etching have been described in detail, the reactor is alsoadvantageous for choices of plasma operating conditions other than thosedescribed above, including different ion densities, different plasmasource power levels, different chamber pressures. These variations willproduce different plasma capacitances, requiring different electrodecapacitances and different electrode-plasma resonant frequencies andtherefore require different plasma source power frequencies and stubresonant frequencies from those described above. Also, different waferdiameters and different plasma processes such as chemical vapordeposition may well have different operating regimes for source powerand chamber pressure. Yet it is believed that under these variousapplications, the reactor will generally enhance the process window andstability as in the embodiment described above.

Compact VHF Fixed Tuning Element:

The coaxial tuning stub of FIGS. 1 and 8 is a tuning element thatprovides an impedance match over a large tuning space, as described withreference to FIGS. 1-7. However, because of its elongate linear design,its footprint is actually larger than that of the plasma reactorchamber. In those situations where this aspect is found to beinconvenient, the coaxial tuning stub of FIGS. 1 and 8 is replaced by anequivalent strip line circuit, as illustrated in FIGS. 12, 13 and 14.The center conductor of the VHF generator 50 Ohm coaxial outputconnector is connected to a strip line conductor 700, while the outerconductor of the VHF generator 50 Ohm coaxial output connector isconnected to the metal ceiling 710 of a housing 715 covering the top ofthe reactor. The conductive ceiling 710 functions as a ground plane thatthe strip line conductor 700 faces. The strip line conductor 700 isgenerally oval in cross-section, with its broader side facing the groundplane ceiling 710. The characteristic impedance of the strip lineconductor is determined by its spacing from the ground plane ceiling710. Preferably, the strip line conductor 700 is uniformly spaced fromthe ground plane ceiling 710 along its entire length.

In an exemplary embodiment, the strip line conductor was 0.125 inch inheight, 2.5 inches wide and is displaced below the ground plane ceiling710 by 0.5 inch. By having the wider (2.5 inch) side of the strip lineconductor 700 facing the ground plane ceiling 710, current flow is moredistributed across the entire 2.5 inch width of the strip line conductor700, thereby reducing resistive losses in the outer surface where mostof the current flow occurs. The length of the strip line conductor 700is determined in the same manner as the length of the coaxial tuningstub 135, as described above in detail with reference to FIG. 1.Furthermore, the placement of the RF tap 160 along the length of thestrip line conductor 700 is also determined in the same manner as theplacement of the RF tap along the length of the coaxial stub 135, asdescribed with reference to FIG. 1. Finally, the end of the strip lineconductor 700 of FIG. 12 furthest from the overhead electrode 125 is,like the corresponding end of the coax stub inner conductor 140 of FIG.1, shorted to ground. In the case of the strip line conductor 700, theshort to ground is achieved by a connection at the far end 700 a to theground plane ceiling 710, as shown in FIG. 13.

Like the coaxial tuning stub 135 of FIGS. 1-8, the strip line conductor700 has a length equal to a quarter wavelength of the resonant frequencyof the fixed tuning element, in this case the strip line circuitcomprising the strip line conductor 700 and the ground plane ceiling.Therefore, the selection of the length of the strip line conductor 700is exactly as the selection of the length of the coaxial tuning stub 135which is described above with reference to FIGS. 1-7. In one embodiment,this length was about 29 inches. The RF tap 160 of FIG. 12 connects theVHF generator to the strip line circuit at a particular point along thelength of the strip line conductor 700, just as the RF tap 160 of FIG. 1makes the corresponding connection along the length of the coaxialtuning stub 135. In the case of FIG. 12, the center conductor of the VHFgenerator output coaxial connector is connected at the tap 160 to thestrip line conductor while the outer conductor of the VHF generatoroutput coaxial conductor is connected to the ground plane ceiling at thepoint overlying the tap connection to the strip line conductor.

The location of the tap point 160 in FIG. 12 along the length of thestrip line conductor 700 is determined in the same manner as thelocation of the tap in FIG. 1 along the length of the coaxial stub, asdescribed above in detail with respect to FIG. 1. With this feature, thestrip line circuit comprising the strip line conductor 700 and theground plane ceiling performs in the same manner as the coaxial tuningstub 135 of FIG. 1, including the feature described with respect to FIG.1 in which the impedance match space can accommodate as much as a 100:1variation in load resistance by slightly offsetting the tap point 160from a theoretical optimum. As described above with reference to FIG. 1,the theoretical optimum location of the tap 160 is at a point along thelength of the tuning stub 135 (or, equivalently, along the length of thestrip line conductor 700 of FIG. 12) at which the ratio between thestanding wave voltage and current equals the output impedance of the VHFgenerator or the characteristic impedance of the coaxial cable connectedtherebetween. The discovery described with reference to FIG. 1 is thatthe impedance match space is surprisingly expanded by offsetting the tap160 by about 5% from the theoretical optimum location. Thus, the stripline conductor circuit of FIG. 12 provides all the advantages andfunctions of the coaxial tuning stub of FIG. 1 but further adds theadvantage of compactness.

Like the inner conductor 140 of the coaxial stub of FIG. 8, the stripline conductor 700 of FIG. 12 is hollow in order to accommodate theutility lines connected to the electrode 125, and is connected to thetop surface 290 a of the conical housing 290. The advantage of the stripline conductor 700 (over the coaxial tuning stub of FIGS. 1 and 8) isthat the strip line conductor 700 can extend in a circular fashionwithin the housing 715 so that its requisite length can be realizedwithout extending beyond the “footprint” of the reactor chamber.

The length of the strip line conductor is determined in the same mannerthat the length of the coaxial tuning stub is determined, as describedabove with reference to FIG. 1. The impedance of the strip lineconductor 700 is determined by adjusting its displacement from theground plane ceiling 710. As described above, this impedance is bestselected to be about 30 Ohms, or less than the VHF generator outputimpedance. The location of the tap 160 from the VHF generator 150 alongthe length of the strip line conductor 700 is made in the same manner asthe location of the RF tap 160 on the coaxial tuning stub as describedabove with reference to FIG. 1. The strip line conductor 700 incombination with the ground plane ceiling 710 performs the same functionas the coaxial tuning stub of FIGS. 1 or 8, and provides the sameperformance advantages as described above with reference to FIG. 1.

While the top view of FIG. 13 shows an embodiment in which the stripline conductor 700 is wound along a nearly square path (with roundedcorners), FIG. 14 illustrates another embodiment in which the strip lineconductor 700 is circularly wound.

Utilities Fed Through the Tuning Element:

As described above with respect to FIGS. 8 and 12, the coaxial stubinner conductor 140 of FIG. 8 and the strip line conductor 700 of FIG.12 are both hollow in order to accommodate lines that carry variousutilities to the overhead electrode. Thus, as illustrated in both FIGS.8 and 12, the outer gas supply line 325 is connected to an outer gasflow controller 800, the inner gas supply line 345 is connected to aninner gas flow controller 810, the optical fiber or light pipe 685 isconnected to the optical detector 687, and the heating/cooling line 675is connected to a heating/cooling source controller 830.

The fixed tuning element 135 is either a coaxial tuning stub (as in theembodiments of FIGS. 1 and 8) or a strip line circuit (as in theembodiments of FIGS. 12 and 14). Antenna designers will recognize theequivalent function performed by both embodiments of the fixed tuningelement in providing an impedance match between the characteristicoutput impedance of the RF generator and the impedance of theelectrode/plasma combination. Both embodiments of the fixed tuningelement (or, equivalently, fixed impedance match element) sharestructural feature in common, including the use of a center conductor(either a strip line conductor in FIG. 12 or an inner coaxial conductorin FIG. 8) and a grounded conductor (the ground plane ceiling of FIG. 21or the grounded outer coaxial conductor of FIG. 8). In both cases, thecharacteristic impedance of the impedance match element is determined bythe spacing between the two conductors, while the input impedance to theimpedance match element is determined by the location along the centerconductor of the connection to the RF generator. Also, the centerconductor is hollow and therefore serves as an RF-shielded conduit forgas feed lines and heat-conductive fluid feed lines. And the mostimportant common feature is that both embodiments of the impedance matchelement are physically fixed in structure, and therefore require nomoving parts or intelligent controllers, a significant advantage. Otherrelated advantages have already been described. The fixed impedancematch element of both embodiments may therefore be referred to ingeneral as a fixed two-conductor impedance match element with a hollowcenter conductor.

While the invention has been described in detail by reference to variousembodiments, it is understood that variations and modifications thereofmay be made without departing from the true spirit and scope of theinvention.

1. A plasma reactor for processing a semiconductor workpiece, comprising: a reactor chamber having a chamber wall; a workpiece support within said chamber for holding the semiconductor workpiece; an overhead electrode overlying said workpiece support; a VHF power generator of RF power at a VHF frequency, said VHF power generator being coupled between said workpiece support and said overhead electrode; and a fixed impedance match element comprising: a larger area ground conductor coupled to a return terminal of said VHF power generator and having a cylindrical portion terminated on said chamber wall; a generally elongate smaller area RF conductor adjacent and spaced from said larger area ground conductor and terminated on said overhead electrode; an RF tap at a location on said generally elongate smaller area RF conductor, said location corresponding to at least a near match between an output impedance of said VHF power generator and a characteristic impedance of said fixed impedance match element, said RF tap comprising a coupling between an RF output terminal of said VHF power generator and said generally elongate smaller area RF conductor.
 2. The reactor of claim 1 further comprising an MERIE magnetic field generator for producing an MERIE magnetic field, wherein said MERIE magnetic field rotates over time across a top surface of said workpiece.
 3. The reactor of claim 2 wherein said MERIE field generator comprises a plurality of electromagnets situated about said chamber wall.
 4. The reactor of claim 2 wherein said MERIE field generator comprises a plurality of permanent magnets situated about said chamber wall.
 5. The reactor of claim 1 wherein said overhead electrode comprises a gas distribution showerhead.
 6. The reactor of claim 2 wherein said MERIE magnetic field enhances uniformity of plasma ion distribution in said chamber.
 7. The reactor of claim 1 wherein said fixed impedance match element comprises a coaxial tuning stub, said larger area grounded conductor comprises a coaxial outer conductor and said generally elongate smaller area RF conductor comprises a coaxial inner conductor inside said coaxial outer conductor.
 8. The reactor of claim 1 wherein said fixed impedance match element comprises a strip line circuit wherein said larger area grounded conductor comprises a planar top and a coaxial side wall, said generally elongate smaller area RF conductor comprises a strip line conductor generally parallel to said planar top and spaced therefrom by a gap corresponding to a characteristic impedance of said fixed impedance match element.
 9. The reactor of claim 5 further comprising a process gas source and at least an elongate gas conduit coupling said gas distribution showerhead to said gas source, said generally elongate smaller area RF conductor being hollow along its length and said elongate gas conduit being inside of and generally following a path of said generally elongate smaller area RF conductor.
 10. The reactor of claim 9 wherein said gas distribution shower head comprises at least an internal thermal control passage and said reactor further comprises at least a thermal control medium source for fluid or gas and at least an elongate thermal control conduit connecting said thermal control medium source to said internal thermal control passage, said elongate thermal control conduit being inside of and generally following a path of said generally elongate smaller area RF conductor.
 11. The reactor of claim 1 wherein said tap is displaced from said location by a fraction of the length of said generally elongate smaller area RF conductor, whereby to optimize an impedance match space of said fixed tuning element.
 12. The reactor of claim 11 wherein the characteristic impedance of said fixed tuning element is different from the output impedance of said VHF power generator by an amount that optimizes a match space between the characteristic impedance of said fixed match element, the output impedance of said VHF power generator and a plasma load impedance of said chamber.
 13. The reactor of claim 1 further comprising a process-compatible layer on a surface of said overhead electrode facing said workpiece support.
 14. The reactor of claim 13 wherein said process-compatible layer comprises silicon-carbide.
 15. The reactor of claim 13 further comprising: plural gas injectors in said overhead electrode facing into said chamber.
 16. The reactor of claim 15 further comprising: an insulating layer between said generally elongate smaller area RF conductor and said overhead electrode.
 17. The reactor of claim 16 wherein said insulating layer provides sufficient capacitance to at least reduce arcing within said plural gas injectors.
 18. The reactor of claim 16 further comprising: an RF bias power source of an HF frequency connected to said workpiece support.
 19. The reactor of claim 18 wherein a capacitance of said insulating layer corresponds to a peak admittance through said overhead electrode and said fixed impedance match element for a harmonic of said HF frequency, whereby to suppress plasma sheath harmonics.
 20. The reactor of claim 15 wherein said gas injectors comprise arcuate narrow slots.
 21. The reactor of claim 20 wherein an individual one of said arcuate narrow slots comprises an annulus.
 22. The reactor of claim 1, further comprising a magnetic field generator situated about said chamber and designed to generate a magnetic field inside said chamber.
 23. The reactor of claim 22, wherein said magnetic field affects the ion distribution of plasma generated in said chamber.
 24. The reactor of claim 22, wherein said magnetic field generator comprises at least one electromagnet.
 25. The reactor of claim 22, wherein said magnetic field generator comprises at least one permanent magnet.
 26. A plasma reactor for processing a semiconductor workpiece, comprising: a reactor chamber having a chamber wall; a workpiece support within said chamber for holding the semiconductor workpiece; an overhead electrode overlying said workpiece support; a first RF power generator for producing RF power at a first frequency and having an output terminal coupled to said overhead electrode; a second RF power generator for producing RF power at a second frequency different from said first frequency and having an output terminal coupled to said workpiece support pedestal; a magnetic field generator for producing a rotating magnetic field in said reactor chamber that rotates over time across a top surface of said workpiece.
 27. The reactor of claim 26 further comprising a fixed impedance match element coupled between said first RF power generator and said overhead electrode.
 28. The reactor of claim 26 wherein a return terminal of said first RF power generator is coupled to said workpiece support.
 29. The reactor of claim 29 wherein a return terminal of said second RF power generator is coupled to said overhead electrode.
 30. The reactor of claim 29 wherein said first frequency is a VHF frequency, said second frequency is an HF frequency.
 31. The reactor of claim 26 wherein said magnetic field generator comprises plural electromagnets situated about said chamber wall.
 32. The reactor claim 26 wherein said magnetic field generator comprises plural permanent magnets situated about said chamber wall.
 33. The reactor claim 27 wherein said fixed impedance match element comprises one of: (a) a coaxial tuning stub, (b) a strip line circuit.
 34. The reactor of claim 27 wherein said fixed impedance match element provides a low impedance path to ground for RF power at said second frequency from said wafer support.
 35. The reactor of claim 34 wherein said fixed impedance match element has a resonant frequency at or near said first frequency of said first RF power generator. 